Standards for most radio-frequency communications are of such stringency that temperature compensation networks are generally necessary to enable crystal oscillator circuits to maintain an established channel frequency over a specified temperature range. Compensation networks, typically including a thermistor, are used to stabilize carrier frequencies F.sub.c of crystal oscillator circuits over typically specified temperature ranges, such as from negative thirty degrees centigrade to positive sixty degrees centigrade.
Several radio frequency transceiver modules for conventional audio transmissions are available from commercial suppliers. Each may differ in interface, timing and analog characteristics. Even though input and output characteristics of transceiver modules may differ, their operating modes are similar. While it might be desirable to adapt available audio-type transceivers to the communication of data in the form of digital signals in binary format, particular interface problems present themselves primarily because of asymmetric signal amplitudes with respect to a quiescent, or steady state, signal level or voltage of a data signal string in comparison to a typical voice communication sequence. Further problems relate to communicating strings of binary digital signals over a radio frequency environment between digital type end user modules, particularly when various transceiver modules of the environment respond to different bias conditions or timing characteristics.
A state of the art FM modulating circuit typically includes a series coupled combination of a capacitor and a varactor diode. The node between the varactor diode and the capacitor is adapted to receive a voltage type signal input for modulating the channel frequency output of the crystal oscillator circuit. Variations in voltages of input signals to the node cause corresponding shifts in the capacitance of the varactor diode and, hence, in the frequency of oscillation of the oscillator circuit. Consequently, the quiescent or steady state voltage at the node constitutes a signal voltage reference at which the crystal oscillator circuit oscillates at its unmodulated channel frequency F.sub.c. Temperature compensation, typically including a thermistor circuit, is applied to the node to compensate for temperature related shifts in performance of the components.
Voice signals typically have no DC component. Such signals can consequently be superimposed through a series-coupled input capacitor on the established, temperature compensated voltage at the node. Without a DC current input to the node the reference voltage at the node and, hence, the channel frequency F.sub.c of the circuit remain stable. If capacitive coupling is not used, and the signal impressed on the input node does include a DC component which differs from the temperature-compensated steady state reference voltage at the node, the reference voltage at the node would be shifted. Such shift, in turn, would shift the channel frequency F.sub.c of the transmission signal with respect to which the modulated signal is centered. Depending on the magnitude of the voltage shift, the modulated frequencies may become shifted beyond allowable limits, such as, for example, those established by the Federal Communication Commission.
Digital data signals typically include a DC component which would tend to cause such signals applied through the typical capacitive coupling to become distorted with what is referred to as "jitter". In state of the art digital signal transmissions, jitter is controlled by electronic transformation or encoding of digital signals. Encoding and subsequent decoding of signals is likely to affect, however, the speed or reliability of data transmission between transceivers.
Consequently, it would be desirable to transmit digital signals over any of various types of traditional voice frequency transceiver units without a need for encoding and subsequently decoding the data signals and not to be concerned about eliminating DC components from a digital data string. It would further be desirable to allow an interfacing of data signals at given signalling rates and signal levels between various types of communication modules by means of any of such various types of voice frequency transceiver units.